Investors have become increasingly alarmed this year about the strength of economies in emerging markets as the Fed begins to scale back its massive monetary stimulus.

Adding to concerns were moves by central banks in Turkey, South Africa and India to raise interest rates as emerging-market currencies tumbled in value.

Fears of a slowdown have been compounded by the Fed's gradual pullback of its stimulus program known as quantitative easing. The central bank began scaling back the monthly purchases of long-term bonds this month.

"This afternoon the Fed will announce whether or not it will further taper its bond-buying program, and U.S. markets are a little jittery as they anticipate the decision's effect on emerging markets," market strategists at ING U.S. Investment Management said in a note Wednesday.

Not only has the Fed's program pushed down borrowing costs by keeping a lid on interest rates, it has also lured investors into riskier assets like stocks. The Fed's stimulus helped lift the S&P 500 30% last year.

“Because investors saw such strong results … they’re looking to lock in profits now, in a sense, by selling first and asking questions later," said Sam Stovall, chief equity strategist at S&P Capital IQ in New York.

As of midday Wednesday, the S&P 500 was off about 3.5% so far this year.

As investors pulled money out of emerging markets, traditional safe havens like U.S. Treasury bonds and gold rose.

Gold futures rose 1% to $1,264.40 an ounce. The yield on the benchmark 10-year U.S. Treasury bond slid to 2.71% from 2.75% the previous day. Bond yields fall when prices rise.